The invention concerns a process for sampling disperse material flows in which, within a process mainstream, an analysis substream is taken from said process mainstream for subsequent analysis, and an apparatus for the performance of this process.
The size distribution of the particles or droplets, hereinafter referred to generally as particles, is of considerable importance for the production of disperse solids and emulsions as it essentially determines the reactivity, transport properties and stability of the material system.
Consequently, knowledge of the particle size distribution (PSD) enables the production process to be optimised and production to be geared to the required quality. To this end, an apparatus for determining the particle size distribution is necessary which can be integrated with ease at various points in the process without causing any noticeable disruption to the process sequence.
Devices for determining particle size distributions have long been well known and employ various measuring principles. In many applications, there is an increasing trend towards devices based on laser diffraction (LD) as this combines high measuring accuracy with good stability of the results, short measuring times, a wide measuring range, a low measuring range bottom limit and easy handling.
These devices utilise the fact that a particle irradiated by a monochromatic, coherent light deflects portions of this light with different degrees of intensity depending on its size, with small particles deflecting the light more intensively than large particles. This deflection of light is known as diffraction.
In a normal arrangement according to FIG. 1, a laser 1 followed by a divergent lens system 2 generates a dilated, parallel measuring light beam which illuminates particles 8 introduced into a measuring cell 7. The diffracted light is directed by a convergent lens 14, the Fourier optical system, onto a photodetector 16 with a multiplicity of elements which, together with a downstream electronics unit, enables the intensity distribution to be precisely mapped. The particle size distribution can then be calculated from this intensity distribution by means of an evaluation unit 20 using known algorithms.
Such an apparatus is equally suitable for determining the particle size distribution of disperse solids, suspensions and emulsions. Known calculation methods based on the Fraunhofer diffraction, yield a particle size distribution irrespective of the optical properties of the particles and those of the surrounding medium.
The suitability of these devices is, however, limited to the range of low particle concentration as operations are preferably performed by transmitted light and the measuring zone must allow the diffracted light to pass through. Moreover, the particle concentration must be kept sufficiently low so that the diffracted light is not diffracted again at downstream particles. This latter phenomenon, known as multiple scattering, can be taken into account when calculating the particle size distribution. The known algorithms for this purpose are limited to special particle forms and require precise knowledge of the optical parameters of the material system, a knowledge that is usually absent in relation to most particles.
The high mass flow rates which are usually encountered in production processes, often amounting to several tons per hour, therefore mean that it is necessary as a rule initially to remove from the process a sample or specimen and to reduce the particle concentration of this sample by addition of the medium surrounding the particles, i.e. by dilution, so that the maximum permissible particle concentration in the measuring zone is not exceeded. This process is only permissible for those material systems in which dilution does not alter the particle size distribution.
The sample in such cases has to be taken in a manner which ensures that the particle size distribution of said sample corresponds to the particle size distribution of the process in the time window under consideration, i.e. such that it is representative. This in turn requires that all areas of the transport cross section are equitably sampled and that the removal of the sample does not change the particle size distribution at the sampling location. If the process particles are incorporated in a flowing medium, then it is well known that the sample has to be removed isokinetically, i.e. that the particles are not allowed to undergo any velocity change as otherwise the particle size distribution of the sample would be noticeably altered as a result.
It is also well known in relation to laser diffraction that agglomerated particles are determined on the basis of their agglomerate diameter. In normal applications, it is the particle size distribution of the primary particles which is of primary interest. The agglomerated particles first have to be separated before they pass through the measuring zone. Various devices are known as being capable of performing this task, termed dispersion, which separate the dry particles in flows of high turbulence by causing the particles to impact against one another or to impact against the walls, or which use specifically introduced obstacles or apply centrifugal forces so that the particles become separated as a result of velocity gradients. Devices are known for suspensions in which a liquid, in some cases assisted by special chemical substances and the application of ultrasound, is used to separate the particles.
In order to eliminate from the calculated particle size distribution fluctuations in the optical properties of the components, e.g. the laser, and fluctuations in the efficiency of the Fourier optical system due to the presence of particles, from time to time a reference measurement is necessary in which the intensity distribution of the medium surrounding the particles is measured in the absence of particles.
Various devices have been proposed for determining particle size distributions within the process, but these only meet the indicated requirements in part, and usually only with considerable restrictions.
In the simplest known type of device, the laser diffraction system, is directly flanged onto the process piping, i.e. the entire process mass flow has to pass through the measuring zone. The high optical concentrations which occur in this case are adjusted on the basis of a material-dependent correction of the multiple scattering pattern. A reference measurement is only possible prior to commencement of a production phase (batch). The light source and detector are kept extensively clean by gas-purged tubes or by an enveloping flow passing along windows. Any contamination of the lens system which still occurs therefore falsifies the results in a protracted manner. Dispersion of the particles does not occur. Such a device is therefore only suitable for very small pipe diameters with production mass flow rates in the range of a few kg/h and where production times (batch times) are short. The analysed sample is rendered non-representative in its definition by the geometry of the measuring zone and the laser beam profile.
In another device, the light source and detector unit are integrated in a rod with an aperture transverse to the rod longitudinal direction, which is immersed through a flange into the process mass flow. The limitations of the above-mentioned type are extended by this non-representative sampling method, with particular problems being encountered in the case of this device with regard to keeping the windows clean.
In a further development, the first-mentioned device type is implemented with static, non-representative sampling in the bypass to a pipe of larger cross section. Sampling is performed at a fixed, definable position in the process pipe. Sample transfer is performed by a jet pump which further dilutes the analysis substream and is adjusted so that the sampling operation is performed as isokinetically as possible. The sampling tube is permanently open and exposed to the abrasive process mass flow. There is no provision for cleaning the sampling aperture. The analysis substream continuously passes through the measuring zone. It is accelerated by the jet pump and diverted several times until it is returned to the process. In order to reduce overall height, 90xc2x0 elbows are used. This arrangement is unfavorable in relation to wear. The wear rate is proportional to the process mass flow and process time, and independent of the number of required measurements. The reference measurement can only be performed at the commencement of a batch.
In the case of another device type, the velocity of the process mass flow is first measured by differential pressure sensors through the incorporation of special flow geometries, and these values are used to adapt the pressure conditions in the analysis substream in order to ensure the isokinetics of the sampling operation. Particle size distribution analysis is performed by a laser diffraction system mounted at the outlet of the pipe, whereby, in one system configuration, the particles are initially separated at a filter and, on attainment of a certain sample quantity, this is measured with a laboratory laser diffraction system. Here, too, the sampling location is static, i.e. not representative of the entire cross section. The actual particle size distribution analysis takes place outside the piping system.
Such a sampling stage was also proposed for the case of through-flow piping with isokinetic sampling. Here again large analysis substreams occur which render sophisticated downstream stages necessary. Here, too, the actual particle size distribution analysis is performed outside the piping.
In order to improve the sampling situation, it was proposed in patent specification DE 35 43 758 C1 that a sampling segment rotate in the process pipe cross section. Where negligible flow occurs, the removal of a representative sample via the drop chute can be ensured. As a rule, the low division ratio renders repeated application of the principle necessary, a fact which, like the necessity of seals between the moving parts in the particle-laden environment, is disadvantageous. The actual particle size distribution analysis occurs outside the piping. To this end, the analysis substream is initially collected in an intermediate accumulator, then transported to the measuring system, whereby the measuring area can be adapted and blockage of the downstream stages avoided by sieving. There then follows a metering operation in a dry disperser, the measuring operation in a laser diffraction sensor, extraction of the aerosol and, where appropriate, return to the process via a cyclone and rotary-vane feeder. This solution has proven extremely effective, but at the same time is both bulky and expensive.
In the field of suspensions, all the solutions introduced to date have foregone the notion of representative sampling. In the devices usually employed for such applications, sampling is performed by a static tube extending into the process piping, or by a pneumatically extractable sampling finger (probe). The sample is conveyed by means of a diluting liquid. The dispersion stage is completely omitted or is performed in an agitated container by means of ultrasound. The diluted suspension is ultimately transferred by cuvette to the measuring zone of a laser diffraction system, and is then disposed of externally or returned to the process. For the reference measurement, the system is switched over to pure liquid by means of controlled valves.
Finally, devices have also been introduced in which other available sampling systems, such as impact samplers, Y valves, sampling screws etc. supply the samples and, once these have been appropriately prepared, analyse them with standard laser diffraction systems. Common to all these devices is that they are only suitable for certain special applications, require frequent maintenance and take up a considerable amount of space.
None of the prior art devices offers universal usability nor the combination of representative sampling for moving media with dispersion, concentration adaptation, a compact design and the possibility of performing a reference measurement with the process running.
The invention is thus based on the problem of providing a sampling method of maximum universal usability, and a compact apparatus for performing the sampling process. It should also offer the capability of representative, continuous sampling even at high production mass flows.
This problem is solved according to the invention by a method and process in which the analysis substream is removed from the process mainstream via an extraction area which is smaller than the cross sectional area of the process mainstream and is defined independently of this process mainstream cross section. During extraction of the analysis substream, the constant extraction area follows an orbital path which sweeps over the process mainstream cross section.
This sampling process according to the invention differs from the prior art methods and processes by virtue of its capability to enable a representative and continuous sampling operation which can also adapt the analysis flows to high production mass flows.
Taking as a basis for comparison the solution according to DE 35 43 758 C1, as it is shown in FIG. 2a, at a mass flow "THgr" (Mass flow {dot over (M)}/Area) in a pipe, which cannot exceed certain values, a larger pipe diameter D is necessary for high mass flows (ROPRON is the trade name for the system shown);. In the solution according to DE 35 43 758 C1 with a rotating sampling segment, this necessarily leads to high analysis mass flows m as the minimum segment angle xcex1 is limited by the maximum particle size or by a multiple of this particle size. The governing equation reads as follows:                     m        .                    =                                            α                          2              ⁢              π                                ⁢                      M            .                          =                                            α                              2                ⁢                π                                      ⁢                          ΘA              Process                                =                                              m          .                ≈                  D          2                                        xe2x80x83                    =                                                          α                              2                ⁢                π                                      ⁢            Θ            ⁢                                          D                2                            4                        ⁢            π                    =                ⁢                  xe2x80x83                                    xe2x80x83                                xe2x80x83                    =                                            α            8                    ⁢          Θ          ⁢                      xe2x80x83                    ⁢                      D            2                          ⁢                  xe2x80x83                                    xe2x80x83            
i.e. the analysis mass flow {dot over (m)} increases with the square of the process pipe diameter D.
FIG. 2b, on the other hand, shows that the likewise prior art solution with a sampling tube is considerably more favorable in this respect. The governing equation in this case is as follows:                     m        .                    =                                                                          A                Analysis                                            A                                  Proze                  ⁢                                      xe2x80x83                                    ⁢                  β                                                      ⁢                          M              .                                =                                                                                                                d                      2                                        4                                    ⁢                  π                                                                                            D                      2                                        4                                    ⁢                  π                                            ⁢                              M                .                                      =                          ⁢                  xe2x80x83                                              m          .                ≈                  d          2                                        xe2x80x83                    =                                                          d              2                                      D              2                                ⁢          Θ          ⁢                      xe2x80x83                    ⁢                      A                          Proze              ⁢                              xe2x80x83                            ⁢              β                                      =                                                            d                2                                            D                2                                      ⁢            Θ            ⁢                                          D                2                            4                        ⁢            π                    =                                    xe2x80x83                                xe2x80x83                    =                                            π            4                    ⁢          Θ          ⁢                      xe2x80x83                    ⁢                      d            2                          ⁢                  xe2x80x83                                    xe2x80x83            
i.e. the analysis substream is now independent of the process pipe diameter and proportional to the square of the selectable diameter of the sampling tube d.
Isokinetic sampling with a static sampling tube is not representative. This problem is now solved by the invention in that the sampling tube travels over the cross section of the process pipe at constant velocity so that the entire cross sectional area or a representative proportion of that area is swept over n times during a measuring period. The orbit curve in this case must be selected so that all the individual zones are only passed once per sampling cycle.
Preferred embodiments of the process according to the invention are defined in claims 2-9.
Particularly advantageous devices for performing the process according to the invention are defined in claims 10-22.
Finally, devices according to the invention can be employed in a particularly advantageous manner for various measuring processes as claimed in claims 23-26.
Devices for determining particle sizes and/or particle size distributions are derived from disclosure herein.